Permanent light coupling arrangement and method for use with thin silicon optical waveguides

ABSTRACT

A trapezoidal shaped single-crystal silicon prism is formed and permanently attached to an SOI wafer, or any structure including a silicon optical waveguide. In order to provide efficient optical coupling, the dopant species and concentration within the silicon waveguide is chosen such that the refractive index of the silicon waveguide is slightly less than that of the prism coupler (refractive index of silicon≈3.5). An intermediate evanescent coupling layer, disposed between the waveguide and the prism coupler, comprises a refractive index less than both the prism and the waveguide. In one embodiment, the evanescent coupling layer comprises a constant thickness. In an alternative embodiment, the evanescent coupling layer may be tapered to improve coupling efficiency between the prism and the waveguide. Methods of making the coupling arrangement are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application No.60/459,349, filed Mar. 31, 2003.

TECHNICAL FIELD

The present invention relates to a light coupling arrangement for usewith thin silicon optical waveguides and, more particularly toarrangements and methods of making silicon-based prism couplers that arepermanently attached to silicon-based optoelectronic arrangements.

BACKGROUND OF THE INVENTION

To meet the bandwidth requirements of current and future high-speedapplications, state-of-the-art telecommunication components and systemsmust provide a host of sophisticated signal processing and routingfunctions, both in the optical and electronic domains. As the complexitylevel increases, integration of more functions and components within asingle package is required to meet system-level requirements and reducethe associated size and cost of the end system. It has been recognizedfor some time that the integrated circuit devices, processes andtechniques that revolutionized the electronics industry can be adaptedto produce optoelectronic integrated circuits. In typical optoelectronicintegrated circuits, light propagates through waveguides ofhigh-refractive-index materials such as silicon, gallium arsenide,lithium niobate, or indium phosphide. The use of high-index materialsenables smaller size devices, as a higher degree of mode confinement andtighter bends may be accommodated. While all transmitter, signalprocessing and receiver functions may be incorporated in a singleoptoelectronic integrated circuit, the system may also be constructedfrom more than one package, in what is defined herein as “multi-moduleintegration ”. Multi-module optoelectronic integration is required whena desired component is incompatible with the integrated device. In somecases, no method of fabricating the component is compatible with thetechnology being considered, while in others it may be impossible torealize the full set of specifications for the component in theintegration platform. Even if all the components are readily achievablein a single integrated device, multi-module optoelectronic integrationwill still be required to meet all user needs. For example, the end usermay require only a limited subset of the transmit/process/receivefunctions, or the user may desire specific or unique physical input andoutput signal configurations.

Multi-module optoelectronic integration is crucial to the success ofsilicon-based optoelectronic integrated circuits. A clear advantage ofusing silicon-based optoelectronic integrated circuits stems from thefact that many required tools, techniques, and processes have alreadybeen developed in silicon to meet the needs of conventional electronics.In addition, the material costs of silicon-based devices areconsiderably lower than those for competing technologies such as galliumarsenide or indium phosphide. However, since silicon-based lasers arejust beginning to be developed, it is not currently possible toincorporate the light source in the same silicon wafer as the signalprocessing and receiver elements. Thus, the light signal must beintroduced to the silicon waveguide from an external source.

One common external source is a separate laser module emitting a freespace beam, followed by optical elements to shape, focus, and steer thebeam, or adjust its polarization state. A second common external sourceconfiguration consists of a fiber-connected (referred to in the art as“pigtailed ”) laser module or another light signal delivered through anoptical fiber, again followed by a similar train of optical elements.While receiving elements may be incorporated in the silicon wafer ason-chip or integrated detectors, there are many applications where theuser will need direct access to the optical signal after the on-chipfunctions have been performed. Thus it is appropriate to provide anoptical output port that would generally be a fiber-based termination,although the preferred embodiments do not exclude other outputconfigurations.

A common prior art technique for coupling light from an external sourceto a silicon waveguide is to cleave end facets on both the waveguide andthe mating fiber termination. Examples of fiber terminations include,but are not limited to: multimode or single-mode fibers with small orzero cleave angles, and specially-shaped or lensed single-mode fibersthat produce spot sizes as small as 1.5 μm. The fiber termination isaligned to allow maximum light transmission through the waveguide, andthen fixed in position. Anti-reflection (AR) coatings can be used onboth the fiber termination and the waveguide facet to reduce the Fresnellosses. Since input and output ports for devices must be located at edgefacets of the waveguide-containing wafer die, significant restrictionson device geometry (e.g., topology and/or size) are imposed by usingthis prior art edge coupling constraint.

The above-described edge coupling technique is effective if themode-field diameter of the desired mode in the waveguide is similar tothe spot size associated with the fiber termination, and if thenumerical apertures (NAs) of the fiber termination and waveguide arewell-matched. However, in many practical applications, siliconwaveguides must be relatively thin, having a thicknesses of less than0.35 μm (with a numerical aperture (NA) essentially equal to 1) toremain single-mode in the vertical direction and enable high-speedelectronic applications. By way of comparison, single mode fibers thatare commonly used for telecommunications applications have mode fielddiameters ranging from 2.5–10 μm, with NAs ranging from 0.1–0 4.Therefore, it is clear that this edge coupling technique is not readilyapplicable for use with relatively thin, sub-micron single mode siliconwaveguides.

As direct coupling in the above-described manner does not provide asufficiently small spot size, alternative techniques to transfer lightinto the waveguide must be used. In one prior art technique, light isincident on a periodic grating structure that may be fabricated throughconventional lithographic techniques. See, for example, Fundamentals ofOptoelectronics, Chicago, Richard D. Irwin, Inc., by C. Pollock, 1995,at pages 309–320.

In a second prior art technique, the beam is incident upon an opticalelement of high-index material that is disposed in very close proximityto the waveguide of interest. One exemplary arrangement of thistechnique is disclosed in an article entitled “Theory of Prism-FilmCoupler and Thin-Film Light Guides” by P. K. Tien et al., appearing inthe Journal of the Optical Society of America, Vol. 60, 1970 at pages1325–1337. In this context, “very close proximity ” is intended to meanthat the separation distance between the optical element and thewaveguide permits evanescent coupling of light from the optical elementto the waveguide. In order for evanescent coupling to occur, the mediumseparating the optical element from the waveguide must have a refractiveindex that is lower than those associated with the optical element andwaveguide materials. In addition, the refractive index of the launchoptical element must equal or exceed that of the waveguide material. Inorder to couple light efficiently from the optical element to thewaveguide for a specified wavelength and waveguide thickness, light mustbe incident on the waveguide at a specific angle of incidence. Toreadily achieve the required angle of incidence, the optical element isfrequently fabricated in the form of a prism. By varying the angle ofincidence of the external beam on the angled facet of the prism, thebeam inside the prism can be refracted at the desired angle. For thisreason, the evanescent technique is generally referred to in the art as“prism coupling ”.

While coupling light into and out of a thin waveguide by launching lightinto and retrieving light from a high-index prism or prisms in closeproximity to the waveguide layer is a well-established technique inlaboratory and waveguide characterization applications, the physicalconnection of the prisms to the waveguide is temporary and notappropriate for a finished device that will be subjected to typical userconditions. Several innovative developments are required before prismcoupling techniques can be effectively utilized to couple a significantamount of light into and out of a device prototype or a finishedproduct.

For example, methods of physically attaching prisms and waveguides toproduce dimensionally stable interfaces with both a high degree ofmechanical integrity and sufficient optical transmission are virtuallyundeveloped. Another key element is to identify and develop appropriatematerials and mass production techniques for prism structures. Eachprism element must be visually inspected and optically tested to verifythat it meets the stringent optical and mechanical specificationsrequired by most prism coupling applications. Thus, current methods ofprism fabrication cannot be readily transferred to efficient productionof device structures requiring multiple input and output ports.Moreover, the high-index material prisms (for example, titaniumdioxide—rutile) used in the prior art for waveguide evaluation cannot beused to evanescently couple light into silicon waveguides due to therefractive index mismatch. Additionally, manufacturable designs andmethods of producing precision prism structures, and appropriate methodsof maintaining the required set of geometrical constraints governing theprism, evanescent coupling layer, waveguide, and input and output beamsover a typical device lifetime remain lacking in the prior art.

Thus, a need remains in the art for a robust technique for evanescentlycoupling light into and out of thin silicon waveguides, with thecoupling arrangement being permanently attached to the optoelectroniccircuit.

SUMMARY OF THE INVENTION

The need remaining in the prior art is addressed by the presentinvention, which relates to a light coupling arrangement for use withthin silicon optical waveguides and, more particularly, to arrangementsand methods of making silicon-based prism couplers that are permanentlyattached to silicon-based optoelectronic arrangements.

In accordance with the present invention, specific embodiments ofpermanently-coupled prism and waveguide structures appropriate for usewith sub-micron silicon waveguides in the wavelength bands of interestfor telecommunications application are disclosed. Specification andcontrol of parameters critical to device operation such as materialchoice, device geometry and associated tolerances, fabrication andstability of the evanescent coupling layer or medium, and beam size,quality, and shape are discussed. Methods of fabricating the inventivedevice that are consistent with the choices and tolerances of thesecritical parameters are given.

In accordance with the present invention, a trapezoidal single-crystalsilicon prism is formed and permanently attached so as to make opticalcontact to an SOI wafer (or any structure including a relatively thin,sub-micron silicon waveguide). Wafer-to-wafer bonding is the preferredmethod for attaching a prism wafer to an SOI wafer. Alternatively,die-to-die or die-to-wafer bonding may be used in performing the presentinvention. Additionally, it is possible to attach the prism wafer (ordie) to the SOI wafer (or die) by bringing the two mating surfaces intoclose proximity or creating optical contact, and thereafter using anadhesive to complete the attachment.

In order to provide optical coupling in accordance with the presentinvention, the silicon waveguide is doped to exhibit a refractive indexthat is slightly less than the prism coupler (refractive index ofsilicon≈3.5). An intermediate evanescent coupling layer, disposedbetween the waveguide and the prism coupler, is formed of a materialwith a refractive index less than both the prism and the waveguide. Inone embodiment, the evanescent coupling layer comprises a constantthickness. In an alternative embodiment, the thickness of the evanescentcoupling layer changes monotonically (along the direction of propagationin the waveguide) to improve the coupling efficiency between the prismand the waveguide. For the purposes of the present discussion, such anevanescent coupling layer will hereinafter be referred to as a “tapered”evanescent coupling layer.

In various embodiments of the present invention, the prism coupler mayinclude a rectangular cavity etched into a prism base surface that iscontiguous with the evanescent coupling layer. For embodiments thatutilize an evanescent coupling layer of constant thickness, the cavitycreates corner edges that function to truncate an incoming beam passingthrough the prism and improve coupling efficiency into the underlyingsilicon waveguide. In one embodiment, the entire base surface of theprism wafer (which may include one or more cavities) is covered with amaterial exhibiting a relatively low refractive index (for example,silicon dioxide). The tapered evanescent coupling layer is used inembodiments that do not require a cavity structure for optical coupling.However, cavity structures may be employed with any of the embodimentsof the present invention to facilitate attachment of the prism couplerto a patterned SOI wafer or die.

The present invention details novel methods of fabricating high-indexprism structures and utilizes silicon-based materials for the prismstructure. Advantageously, precision silicon-based prism structures canbe produced in quantity by well-known, wafer-scale silicon processingtechniques. Novel methods of controlling the critical interface betweenthe prism structure and the waveguide are introduced through the use ofappropriate die-to-die, die-to-wafer or wafer-to-wafer alignment andbonding techniques. The inventive method of achieving high-efficiencycoupling in thin silicon waveguides fabricated by silicon-on-insulatortechnology is made possible by these designs and techniques.

Other features and advantages of the present invention will becomeapparent during the course of the following discussion and by referenceto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIG. 1 illustrates an exemplary prior art prism coupling configurationutilized for waveguide characterization;

FIG. 2 illustrates another prior art waveguide characterizationarrangement, this embodiment using a pair of prism couplers;

FIG. 3 illustrates a conventional multi-layer SOI wafer structure,including a relatively thin silicon waveguide layer;

FIG. 4 contains a first embodiment of a silicon-based prism couplingarrangement formed in accordance with the present invention;

FIG. 5 contains an alternative embodiment of a silicon-based prismcoupling arrangement of the present invention including etch-createdareas on the die or wafer to permit optical or electrical measurement,test and inspection activities;

FIG. 6 is an alternative arrangement of the embodiment of FIG. 4,including a low refractive index coating layer disposed over the cavityportion of the prism structure;

FIG. 7 is yet another alternative arrangement of the embodiment of FIG.4, in which an anisotropic wet etch process is used to create angledcavity walls;

FIG. 8 includes a second embodiment of a silicon-based prism couplingarrangement of the present invention, utilizing a pair of prism couplersas separate input and output couplers;

FIG. 9 illustrates yet another embodiment of the present invention,utilizing a tapered evanescent coupling region with a linearly varyingthickness disposed between the thin silicon waveguide layer and theprism coupler;

FIG. 10 is an exploded view of the input portion of the arrangement ofFIG. 9;

FIG. 11 illustrates an alternative to the arrangement of FIG. 9, whereinin addition to the tapered evanescent coupling region, a cavity isincluded in the prism coupler to accommodate various topology variationsin the SOI wafer;

FIGS. 12–19 illustrate various major steps in an exemplary process offorming a prism coupler structure in accordance with the presentinvention;

FIG. 20 is a graph of the input and output beam profiles associated withusing a constant thickness evanescent coupling region;

FIG. 21 is a plot of coupling efficiency as a function of evanescentcoupling layer thickness, for three different silicon waveguide layerthicknesses; and

FIG. 22 is a graph of the input and output beam profiles associated withusing a tapered thickness evanescent coupling region.

DETAILED DESCRIPTION

FIGS. 1 and 2 illustrate two well-known examples of prior art prismcoupling arrangements that utilize the principle of evanescent couplingto introduce a light signal into a planar waveguide structure. In thearrangement of FIG. 1, a light beam is introduced from an externalsource to an input facet 10 of a prism 12 and thereafter propagatesthrough prism 12, passes through an evanescent coupling medium 13 (inthis case, air) and then into a waveguide 14. The light signal is thenextracted from waveguide 14 by evanescently coupling light back out ofwaveguide 14 and into prism 12. Light is detected from output facet 16of prism 12 via a photodetector 17 only when the angle of incidencefulfills the phase-matching condition determined by: (1) thepolarization state of the input light signal; (2) the refractive indexesof prism 12, evanescent coupling medium 13, waveguide 14 and waveguidesubstrate 19; (3) the thickness (g) of evanescent coupling medium 13 andthe thickness of waveguide 14; and (4) the order of the mode in thewaveguide. By detecting light for two different modes, it is possible touse this configuration to extract information on the refractive indexand thickness of the waveguide. In this prior art configuration, anactuator 21 is used to adjust the gap g between prism 12 and waveguide14.

In the arrangement of FIG. 2, two separate prisms 20 and 22 are used toevanescently couple light into and out of a waveguide 24. A light beamis introduced from an external source to an input facet 26 of inputprism 20 and then passes into waveguide 24 via evanescent couplingregion 25. The light beam propagates along a predetermined length ofwaveguide 24 and is then coupled into output prism 22 and exits throughoutput facet 28 of output prism 22. By varying the prism-to-prismdistance, additional information can be extracted on the attenuation perunit length of the waveguide structure. As output prism 22 is typicallyseparated from waveguide 24 by a few hundred nanometers, index-matchingfluid 29 is generally used to facilitate moving output prism 22laterally over the wafer to perform the measurement. Additionally,index-matching fluid 29 serves as the evanescent coupling medium and,therefore, must exhibit a refractive index and viscosity value thatenables optical coupling. In this case, a first actuator 31 is used tocontrol the gap spacing between input prism coupler 20 and waveguide 24,while a second actuator 33 is used to control the gap spacing betweenoutput prism coupler 22 and waveguide 24.

In the arrangements of both FIGS. 1 and 2, the motion of actuators 21,31 and 33 must be extremely well-controlled. In particular,displacements in a direction normal to the mating prism and waveguidesurfaces (i.e., motion that varies the thickness g as illustrated inFIG. 1) must be changed in nanometer-scale increments to optimizecoupling efficiency. Such fine motion control requires the use ofexpensive and bulky piezoelectric drivers or stages. It is alsoimperative that the displacement is confined entirely to the normalaxis; any motion along axes orthogonal to the normal will introduce awedge angle between the mating surfaces. In addition, if the wedge angledoes not permit the gap in the evanescent coupling region to assume asufficiently small value (thus characterizing “very closeproximity”—where this may occur if regions of the prism base physicallycontact the waveguide), no significant amount of light will be coupledthrough the arrangement.

For output prism 22 of FIG. 2, an additional actuation mechanism (notshown) is required to translate the position of output prism 22vis-à-vis input prism 20. Again, the lateral displacement of outputprism 22 must be confined entirely to a single axis to prevent wedgeformation and the associated decoupling problems. Lastly, the physicalsize and extent of actuators 31 and 33 can limit the minimum separationdistance that can be achieved between input prism 20 and output prism22, which presents a problem when it is desired to couple into shortsections of waveguide. Thus, significant improvements, both in couplingefficiency and ease of use, are made in accordance with the presentinvention by physically attaching a coupling prism to a wafer/diecontaining a waveguide.

Both examples of the prior art utilize evanescent prism coupling toenable waveguide characterization. In this application, the prism orprisms, the input light source and input optics, the actuation mechanismto bring the prism or prisms into close proximity to the waveguide, aswell as the detection system, are all defined as components of theprism-coupler system. Such systems are temporarily coupled to a givenwaveguide for evaluation purposes only; the waveguide is then decoupledand subsequent waveguides for evaluation are introduced to the prismcoupler system. The arrangement of the present invention overcomes the“temporary” nature of prior art coupling systems.

The various embodiments of the present invention all concern evanescentcoupling of light into thin silicon waveguides via silicon prismstructures that are produced by a combination of conventionalsemiconductor processes and materials methods includingsilicon-on-insulator, lithography, etching, thin-film deposition,anti-reflective coating, and wafer-to-wafer attachment/bondingtechnologies. These silicon waveguides are required to be relativelythin, “sub-micron”, on the order of 0.35 μm or less, so as to be able tosupport a single mode light signal in the vertical direction. In thehorizontal direction, single or multi-mode operation may be supported bychoosing an appropriate width for the waveguide. Thus, during theremainder of this discussion, the waveguide may be referred to as a“single mode silicon waveguide”, with the understanding that a multimodewaveguide and/or a thicker waveguide may also be used and supported. Theterm “silicon waveguide”, as used in the specification, is notconsidered to be limiting in the definition of the waveguide as a singlelayer of single-crystal silicon material. The “silicon waveguide” of thepresent invention may also comprise a multilayer construction of siliconand/or polysilicon materials with specific characteristics for eachindividual layer (i.e., doping and thickness) tailored for a variety ofdifferent device applications. In some embodiments, the silicon-basedlayers are separated by extremely thin dielectric layers (on the orderof 100 Å or less) that still permit a high degree of coupling betweenthe silicon-based layers within the waveguide structure.

A simple example of a conventional multilayer structure 30 of a wafercontaining a thin silicon waveguide layer 32 that may be used inaccordance with the present invention is shown in FIG. 3. The basesubstrate consists of a silicon layer 34 that is typically 500–1000 μmin thickness. Directly on top of silicon layer 34 is a buried oxide (orBOX) layer 36 that typically consists of 0.4–3.0 μm of silicon dioxide(or other suitable insulator material). For non-optical electronicapplications, BOX layer 36 is required to achieve electrical isolationof elements in the active silicon layer 32 from the bulk silicon layer34. For optoelectronic applications, such as in the case of the presentinvention, BOX layer 36 also serves as the low-refractive index mediumand bottom cladding layer (n≈1.45 for silicon dioxide in the 1.1–1.6 μmregion) that permits total internal reflection (TIR) at the base ofsilicon waveguide layer 32. In order for light to be single-mode withina high refractive index material such as silicon (n≈3.5 in the 1.1–1.6μm region), the thickness of silicon waveguide layer 32 should notexceed 0.35 μm. In the particular embodiment as illustrated in FIG. 3,an additional thin silicon dioxide layer 38 is deposited on top ofsilicon waveguide layer 32 as a protective covering layer. The thicknessof layer 38 may vary across the wafer, but needs to have a prescribedthickness in the optical coupling regions. If the prescribed thicknessin the optical coupling regions is nonzero, layer 38 forms at least partof the evanescent coupling layer. In other sections of the wafer (notused for optical coupling) the thickness of layer 38 may be increased,so as to serve as an upper cladding layer for the optical waveguide 32,as well as to provide electrical isolation from conducting wires thatmay be employed in the device. Total internal reflection (TIR) withinwaveguide layer 32 is ensured by the use of a material for layer 38 witha relatively low refractive index, such as silicon dioxide (n=1.45),silicon nitride (n≈2.0) or air (n=1.0).

In accordance with the present invention, to couple light efficientlyinto the multilayer structure of FIG. 3 throughout the device lifetime,the inventive prism structure is permanently attached to a wafer (or adie portion of a wafer) containing a waveguide structure such aswaveguide 32. During the attachment process, the thickness of theevanescent coupling layer must be very tightly controlled in magnitudein order to obtain optimal coupling efficiency. Subsequent to theattachment process, the optimal coupling efficiency must be maintainedover the device lifetime. FIGS. 4–11, as will be discussed in detailbelow, all illustrate particular embodiments of the present inventionthat accomplish this goal. It is to be understood that the preferred“attachment” mechanism is bonding; however, other adhesives and meansfor providing permanent attachment of the silicon prism coupler to awafer/die may be used. For these alternative joining methods, thephysical attachment of the prism coupler to the SOI wafer or die mayoccur at surfaces removed from the evanescent coupling region.

To achieve high coupling efficiency, the prism structure of the presentinvention is defined by the following characteristics:

1) The refractive index of the prism is equal to or greater than that ofthe silicon waveguide;

2) The prism material exhibits a relatively high optical transmittancein the wavelength range of interest (e.g., 1.1–1.6 μm);

3) The base surface of the inventive prism that mates with the SOI wafermust exhibit a high degree of smoothness. If this condition is not met,the resultant thickness of the evanescent coupling layer will bedetermined by “high points” on the prism surface, instead of the desiredrange of values that guarantees high coupling efficiency;

4) In some embodiments, as will be described below, a well-defined edgeor corner must be fabricated that sharply truncates the input beam topermit optimization of coupling during alignment. For the purposes ofthe present invention, the term “well-defined” means that the dimensionsof typical irregularities at that edge must be much smaller than thedimensions of the projection of the input beam upon the mating orcoupling surface of the prism; and

5) High-quality anti-reflection coatings are preferred on the angledfacets of the prisms to prevent the significant Fresnel losses thatoccur when a light beam is incident from air upon a high-refractiveindex medium, or vice versa.

As the prism structures of the present invention will be permanentlyattached to the waveguide wafer, it is important that the prismstructures embodied by the above-defined characteristics be fabricatedreliably and reproducibly in a material that is readily obtainable.Suitable embodiments of such prism structures are illustrated in FIGS.4–11.

As mentioned above, prior art prism coupling arrangements have not beenefficient in terms of coupling energy into silicon waveguides (even inthe case of temporary coupling arrangements), since prisms with asufficiently high refractive index (n≈3.5 or higher) are notcommercially available. For example, in most prism couplers, thematerial with the largest refractive index that is used is titaniumdioxide crystal (rutile), which has an extraordinary refractive indexn_(e)=2.8 in the visible portion of the spectrum, and a value closer ton_(e)=2.6 in the optical spectrum of interest to the present invention(1.1–1.6 μm). Traditional prior art optical glasses exhibit an evenlower refractive index, generally on the order of n≦2.1. One possiblealternative is to use prisms of GaAs, which exhibit a refractive indexof n≈3.6 in the optical wavelength range of interest. However, prisms ofGaAs are not readily manufacturable, since GaAs is a relatively brittleand expensive material.

In accordance with the present invention, therefore, a solution to thisprior art problem is to use very similar single-crystal siliconmaterials for both waveguide layer 32 and the prism coupler.Single-crystal silicon prisms, as will be discussed below, can bereadily and inexpensively manufactured using the same processing andfabrication techniques used to form other silicon-based electronic andoptical circuits and devices. Advantageously, the optical transmittanceproperties of the single-crystal silicon prism are very high in thetelecommunications wavelength bands of interest, as is the case for thesilicon waveguides, where it is known that optical loss is less than 0.4dB/cm in a silicon waveguide structure. For optimal device operation, itis preferred to utilize a silicon prism wafer with a refractive indexthat slightly exceeds that of the device silicon layer. As mentionedabove, in order to satisfy the mode-matching condition from the prism tothe waveguide, the prism refractive index must equal or exceed that ofthe waveguide. However, there will be a Fresnel reflection (thusreducing the amount of transmitted light) that depends on the refractiveindex mismatch between the silicon prism wafer and the silicon devicelayer of the SOI wafer. Thus, to fulfill the mode-matching condition butalso minimize the transmission loss through the device, the refractiveindex of the prism wafer should be chosen to be slightly greater thanthe silicon waveguide layer.

One advantage of the present invention is that the desired relationshipbetween the refractive indexes of the silicon prism coupler and thesilicon waveguide layer within the SOI structure can be easily achievedby specifying appropriate ranges of resistivity values for the twosilicon materials. For example, in a standard SOI wafer, the siliconwaveguide layer can be doped with boron, aluminum, or gallium to producea p-type material; or with phosphorus, arsenic, or antimony to producean n-type material. Available silicon wafer resistivity levels cover therange from 0.001 Ω-cm (which corresponds to a doping level of 10²⁰/cm³at room temperature) to 10,000 Ω-cm (which corresponds to a doping levelof 10¹²/cm³ at room temperature). As an example, for a commercial gradeSOI wafer that has undergone all the steps associated with SOI waferprocessing, the device silicon waveguide layer of a “nominally undoped”<100> SOI wafer has a resistivity value on the order of 10–20 Ω-cm(approximately 10¹⁵/cm³ doping level at room temperature). By specifyingan appropriate minimum resistivity value for the silicon prism wafer (inthis example, a minimum resistivity exceeding 20 Ω-cm) and the type ofdopant, it is ensured that the refractive index value of the siliconprism wafer will equal or exceed that of the device silicon waveguidelayer of the SOI wafer. By specifying a maximum silicon prism waferresistivity of approximately 1000 Ω-cm (approximately 10¹³/cm³ dopinglevel at room temperature), the refractive index of the silicon prismwafer will exceed that of the silicon device layer of the SOI wafer byabout Δn≦10⁻⁵.

FIG. 4 illustrates a first embodiment of a prism coupling arrangement ofthe present invention. In this particular example, a single-crystalsilicon trapezoidal prism 40 is used to provide both input and outputcoupling between an exterior beam and single mode silicon waveguide 32.In this case, single mode waveguide 32 has a thickness of onlyapproximately 0.14 μm. While this choice of thickness is an exemplaryvalue, it is to be understood that the thickness of the waveguidinglayer is not so limited. An AR coating layer 42 (such as siliconnitride) is disposed to completely cover the exterior surface oftrapezoidal prism 40. Since silicon has a refractive index ofapproximately 3.5 for the wavelength range of interest, and the mediumexternal to prism 40 is air (n=1.0), the Fresnel loss associated witheach transit through an air-silicon interface is on the order of 30%. Astwo such transits are required to complete the path through any givendevice, a 50% loss would occur at the prism surfaces alone, where thisamount of loss is unacceptable for most applications. Thus, the presenceof AR coating 42 serves to significantly reduce the Fresnel loss. In oneembodiment, AR coating 42 may comprise a layer of silicon nitride ofappropriate thickness, which has a refractive index on the order of 2.0.Other materials, or combinations of materials (in one or more layers)may also be used.

Referring back to FIG. 4, base 44 of prism 40 is shown as disposed abovedielectric cladding layer 38. As mentioned above, layer 38 functions asthe evanescent coupling layer for this inventive structure. Inaccordance with this particular embodiment of the present invention, anetched-back cavity 46 is formed through base 44 of prism 40, cavity 46formed to provide comer edges 48,50 that facilitate the truncation ofthe light beam at the interface between prism 40 and layer 38.

As shown in FIG. 4, an input beam I passes through an input facet 52 ofprism 40 and truncates at comer edge 48, where it is coupled throughevanescent coupling layer 38 into silicon waveguide layer 32. Afterpropagating through waveguide 32, an output beam O will encounter comeredge 50 and thereafter be coupled back into prism 40 via evanescentcoupling layer 38 and ultimately exit prism 40 along output facet 54. Aswill be described in detail hereinbelow, input facet 52 and output facet54 are preferably formed using an anisotropic wet etch process (that is,a process that etches different crystal planes at different rates). Fora silicon wafer with a <100> crystal orientation, anisotropic wetetching produces characteristic V-groove structures, with sidewalls thatform a 54.74° angle with respect to the plane of the wafer (as shown inFIG. 4). To improve the optical quality of input facet 52 and outputfacet 54, an additional isotropic etch step may be used to reduce thesurface roughness.

In the particular embodiment of FIG. 4, etched cavity 46 comprises anair gap surrounded by bare silicon sidewalls, where air exhibits arefractive index n=1.0. The beam is required to be truncated at inputcomer edge 48 to prevent light that has been transferred from prism 40to waveguide 32 from being coupled back into prism 40 via the samemechanism. The sharpness of the truncation is important in that it willdetermine the magnitude of the coupling efficiency. As will be discussedbelow, a reactive-ion etch process can be applied to a masked wafer toproduce both the required corner edge and a short section of verticalwall. Other etching techniques, such as ion milling, plasma etching, wetchemical etching, or the like, may also be used to define the comeredges of the cavity within the inventive prism coupling structure.

FIG. 5 illustrates an alternative to the embodiment of FIG. 4, where anadditional window 60 is created at a location along top surface 61 ofprism 40 such that an anisotropic etch process may be used to removeadditional silicon material from the interior portion of prism 40,forming in this example interior edge surfaces 62, 64. If the etchcontinues until a portion of base surface of prism 40 is removed, theunderlying region of SOI wafer will be exposed. As mentioned above,etched windows permit direct viewing of, as well as physical contact to,the underlying SOI wafer 30 in areas where visual inspection, specificoptical tests and/or electrical testing is required. In the particularembodiment of FIG. 5, window 60 is opened in approximately the midportion of trapezoidal prism 40 to allow physical contact to the devicestructure in waveguide layer 32 located immediately below cavity 46.Such an arrangement is exemplary only, and it is to be understood thatwith an appropriate mask and etch process that any desired area of SOIwafer 30 can be accessed in this manner, provided that the window is notimmediately adjacent to an input/output facet structure. In thisparticular arrangement as shown in FIG. 5, both prism facets 52, 54 andwindow 60 are produced using similar anisotropic wet chemical etchprocesses. However, one or more windows may be produced using, forexample, separate reactive ion etch processes, in that case formingvertical (rather than angled) walls in the window area.

Another embodiment of the present invention is illustrated in FIG. 6,where cavity 46 is shown as covered with a relatively thin layer 56 of amaterial exhibiting a relatively low refractive index, for example,silicon dioxide (n=1.45). In this context, “low refractive index”corresponds to a value that will provide total internal reflection atthe silicon/coating interface for a typical range of angles of incidencefor input beams I, and is typically less than 2. Exemplary materialsthat meet this criterion (apart from silicon dioxide) include siliconnitride, silicon oxynitride and silicon carbide. As will be discussedbelow, for device configurations in which the material or materials thatconstitute the evanescent coupling region are deposited or grownpartially or completely upon base surface 44 of silicon prism 40 priorto attachment, the etched surfaces of cavity 46 and base surface 44 willsimultaneously be coated with the same material. That is, layer 56 maybe automatically created in the same process step used to deposit layer38. One advantage of the embodiment of FIG. 6 is that all surfaces ofprism 40 can be coated, protecting prism 40 from environmental hazards.In this particular embodiment, a reactive-ion etch process may beapplied to a masked prism wafer to form both required comer edges 48, 50and a short section of vertical wall 51, 53 prior to the above-describeddeposition step.

FIG. 7 illustrates a slight variation of the embodiment of FIG. 6, wherein this case cavity 46 is formed using an anisotropic wet etch processsimilar to that used to form facets 52 and 54, in place of the reactiveion etch process described above. In some instances, wet etching may bea preferred method of creating cavity structures, in order to reduce thetotal number of different processes required for prism fabrication. Asshown in FIG. 7, the use of a wet etch process yields angled sidewalls66, 68, while still maintaining the required comer edges 48 and 50. Thatis, since angled sidewalls 66, 68 rise steeply from the plane of SOIwafer 30 (i.e., at an angle of 54.74°), the abrupt discontinuity at base44 of prism 40 will naturally form the desired comer edges 48, 50. Theparticular embodiment of FIG. 7 also includes thin layer 56 as describedabove. However, it is to be understood that the use of ananisotropically etched cavity 46 may be used without such a coveringlayer, as is shown in the arrangements of FIGS. 4 and 5.

An advantage of the prism coupling structure of the present invention isthe inherent compactness in the arrangement. The small footprint isenabled by the precise patterning available through lithographictechniques and by the high degree of alignment accuracy between (1)multiple prism structures, and (2) prism structures and the waveguide.In particular, base 44 of trapezoidal prism 40 may be short as 1–2 mm.However, since cavity 46 must be formed so as to bridge the full lengthof the device structure in the underlying silicon waveguide 32, base 44is more likely on the order of 1–10 mm. By contrast, prior artarrangements that utilize adiabatic tapers to couple light from fiberinputs and outputs to thin silicon waveguides require tapers that areapproximately 3 mm in length to couple to waveguides with thicknesses onthe order of 3–5 μm. As tapers are required on both the input and outputports of such a prior art device, the minimum length of the lightcoupling structure using adiabatic tapers is approximately 6 mm. Thus,the number of devices that can be produced on a wafer of a fixed size isincreased by using the smaller dimensioned arrangement of the presentinvention. It is anticipated that the density of devices per unit areasupported by the present invention will continue to increase withimprovements in silicon manufacturing and bonding technologies.

FIG. 8 illustrates an alternative embodiment of the present inventionthat utilizes a pair of trapezoidal prisms 70, 72 to provide couplinginto and out of silicon waveguide layer 32. An AR surface coating 74 isdisposed to completely cover both input coupling prism 70 and outputcoupling prism 72. Depending on the exact sequence of process steps, theAR coating may cover the SOI wafer region between trapezoids, as shownin FIG. 8, or these wafer regions may remain free of the AR coatingmaterial or materials. An input beam I is coupled through input facet 76of input prism 70 and propagates through prism 70 in the manner shown.When input beam I reaches corner termination 78 of prism 70, beam I willbe coupled through evanescent coupling layer 38 into waveguide layer 32.Since the opposing corner 78 of prism 70 will truncate the beam andprovide coupling, there is no need to form an etched region in theinterior of prism 70 along base 80 for beam truncation purposes, as wasthe case for the embodiments described above in association with FIGS.4–7. In a similar manner, edge 82 of output prism 72 will couple thelight beam out of waveguide layer 32 and into prism 72, directing theoutput light beam O to exit along output facet 84 of output prism 72.The structure as illustrated in FIG. 8 is considered to be functionallyequivalent to the above-described prism structures, provided that corneredges 78 and 82 are sharp and well-defined, applying the same criteriaas mentioned above. While not depicted in FIG. 8, etched cavitystructures may still be required with this embodiment, or any embodimentof the present invention, to provide necessary physical clearance aboveSOI wafer 30, as well as to properly mate and attach the prism wafer ordie to the SOI wafer or die.

In each of the embodiments of FIGS. 4–8, evanescent coupling layer 38 isdefined as a silicon dioxide layer disposed directly above waveguidelayer 32 on SOI wafer 30. In the prior art, the evanescent couplinglayer was an air or liquid-filled region that temporarily separated thewaveguide from the prism coupler during the evaluation process. For thepermanent coupling arrangements of the present invention, a reliable andstable attachment, preferably in the form of a bond, must be formedbetween the base surface of the silicon prism(s) and the waveguidesurface of the SOI wafer. Advantageously, evanescent coupling layer 38may be used to provide this desired attachment property. In general,evanescent coupling layer 38 can comprise any medium/material with asufficiently low refractive index. For the purposes of the presentinvention, “sufficiently low” refractive index means that the modepropagation constant is imaginary in the evanescent coupling layer 38,but real within the silicon prism and waveguide layer 32. If thiscondition is satisfied, the electric and magnetic fields decayexponentially across the evanescent coupling layer thickness. Forexample, using the mode angles and propagation constants appropriate tosilicon prisms and thin silicon waveguides, a “sufficiently low” indexmeans less than approximately 2.4. In addition, for the variousembodiments as discussed above it is desirable that evanescent couplinglayer 38 be compatible with conventional semiconductor processingtechniques. Therefore, air (n=1.0), silicon dioxide (n=1.45) and siliconnitride (n=2.0) are all examples of appropriate evanescent couplingmedia that may be used in accordance with the teachings of the presentinvention.

FIG. 9 illustrates yet another alternative embodiment of the presentinvention, where a prism coupler 90 including an input coupling facet 92and output coupling facet 94 is utilized with an evanescent couplinglayer 96 that exhibits a linearly varying thickness, as shown. FIG. 10is an exploded view of the input portion of the arrangement of FIG. 9,depicting a set of three rays 93, 95 and 97 that loosely define thedimensions of the input beam. Ray 93, the left-most (or lowest) ray ofinput beam I, is refracted to the region 96-L of evanescent couplinglayer 96. Ray 95, the center ray of input beam I, is refracting tocentral region 96-C of evanescent coupling layer 96 and ray 97 (theuppermost ray) is refracted to upper region 96-U of evanescent couplinglayer 96. Light is first coupled into region 96-L, with more and morelight accumulating in the underlying silicon waveguide layer 32 as thelight progresses from region 96-L to region 96-C and then into 96-U. Thethickness variation is controlled such that the evanescent couplingthickness is thinner than the optimal value in region 96-L and thickerthan optimal in region 96-U. Here, the term “optimal” is used to denotea thickness essentially equal to the value that maximizes couplingefficiency for an evanescent coupling layer of constant thickness. Ifthe magnitude of the thickness variation is properly controlled, it isnot necessary to form a cavity in the optical coupling region of prism90, since the tapered evanescent coupling layer performs the samefunction of decoupling the light from prism 90 and into waveguide 32 (orvice versa at the output). This occurs since the thickness of taperedevanescent coupling layer 96 is sufficient large in the region beyond96-U to prohibit light from coupling from waveguide layer 32 back intoprism 90.

Although cavity structures are not required to abruptly cut off the beamin the optical coupling areas when a tapered evanescent coupling layeris utilized, similar structures may still be required to providephysical clearance above SOI wafer 30, as well as to properly mate andattach the prism wafer/die to the SOI wafer/die. As discussed above,thin dielectric layer 38 (see FIG. 3) that immediately overlies thesilicon waveguide may vary in thickness from one portion of the wafer toanother. In general, the regions of layer 38 that mate to the opticalcoupling surface are thinner than other regions of layer 38, so that thenon-optical coupling regions of layer 38 form “high points” (or moreaccurately, “high surfaces”) on SOI wafer 30. If layer 38 is directlymated to a prism wafer or die with a completely flat base surface, theoptical coupling surfaces of the prism wafer and the SOI wafer will notmake proper physical contact and therefore cannot be permanentlyattached in the manner required for the coupling arrangement of thepresent invention. In the above-described embodiments, the attachmentprocess is successful because the formation of a cavity in the basesurface of the prism is formed to exhibit a height of several tens ofmicrons, automatically providing the required clearance. That is, theattachment process works because the prism wafer has a patterned, notflat, base surface. The formation of cavity structures in the prismcoupler to accommodate the topology of the SOI wafer may be employedwith any of the embodiments of the present invention, not only thatdisclosed in FIG. 11.

FIG. 11 depicts an arrangement similar to FIG. 9, except that slab SOIwafer 30 of all previous embodiments is replaced with a device SOI wafer100 in which surface layer 102 exhibits a non-planar (or patterned)surface structure in the vicinity of the device structure in anunderlying silicon waveguide layer 104. For the purposes of discussion,surface layer 102 is illustrated as having two different thicknessvalues, d₁ and d₂, where d₁ is defined as the thickness in the opticalcoupling region and d₂ is defined as the thickness in device regions. Inmost cases, d₁ has a value ranging from zero to d₂ and d₂ is typically afew microns greater than d₁. In this coupling arrangement, a prismcoupler 110 including a cavity 112 is used to couple light into and outof waveguide layer 104. The evanescent coupling layer 114 is thendeposited over base surface 116 of prism 110. If cavities 112 aresufficiently deep, the dielectric material that accumulates on regionsof the prism surface that are not associated with the taper will provideadequate clearance for the “high surfaces” of layer 102 on SOI wafer100.

FIGS. 12–19 illustrate various major steps in an exemplary process offorming a prism coupler structure in accordance with the presentinvention. The process steps are considered to be exemplary only, andthe present invention is not considered to be limited to any particularprocess step choices. Referring to FIG. 12, the prism fabricationprocess begins with a silicon substrate 200 that is oriented along the<100> crystal plane. In accordance with the present invention, siliconsubstrate 200 is chosen to have a dopant species and concentration suchthat the refractive index of the prism wafer is at least equal to thatof the silicon waveguide in an associated SOI wafer of the finalstructure (not shown). The thickness of substrate 200 is selected tofabricate a prism coupler with a specific height. Advantageously, aprism design based on industry standard wafer thickness (e.g., 625 μm±25μm for a six-inch diameter wafer) will result in significant costreduction. Base surface 210 of substrate 200 is desired to exhibit avery smooth surface to facilitate the attachment (e.g, bonding) of thecompleted prism wafer to the SOI waveguide wafer.

As shown in FIG. 12, a thermal oxide 212 is first grown on substrate 200to function as a hard mask during the process of defining the cavityregion (particularly required when using some deep RIE processes). Ahard mask is typically defined as a material that is more resistant to asubsequent etch process than the photoresist conventionally used withstandard lithography processes. Subsequent to growing oxide 212, a mask214 is used to define layer 212 and (ultimately) cavity region 216, asshown in FIG. 13. As discussed above, cavity region 216 may be formedusing an RIE process, an anisotropic wet chemical etch process, oralternatively using processes such as plasma etching or ion milling. Asdiscussed above, sidewalls 218 of cavity 216 are then used to define thecomer edges for the prism coupler. Hence, sidewall roughness that issignificantly less than the wavelength of the optical beam is requiredfor the sidewall region that is in the beam path. Subsequent to theformation of cavity 216, the remaining portions of oxide 212 areremoved, as shown in FIG. 14, such as by using a buffered HF etchant.

Once cavity region 216 is formed, substrate 200 is re-oriented such thatopposing surface 230 may be processed to form the required couplingfacets. FIG. 15 illustrates substrate 200, as covered by an etch mask220 to define the locations of the prism facets, where back-to-frontalignment may be used to confirm that facets are properly formed withrespect to the opposing cavity region. Surface 230 is then patterned, asshown in FIG. 16, to expose the desired areas of substrate 200 and ananisotropic wet etch process (using, for example, KOH) is used to form aset of V-grooves 222 (at an angle of 54.74° with respect to thehorizontal direction). The depth of V-grooves 222 is determined by theproper selection of the openings in masking layer 220. It should benoted that alignment of the photolithography mask to thecrystallographic axis of substrate 200 is highly desirable for producingsmooth V-groove surfaces . To further improve the surface smoothness ofthe etched V-grooves, a wet isotropic polish etch may be used. It is tobe noted that a through-etched window, as discussed above in associationwith FIG. 5, can also be etched in the same processing step using alarger opening on mask layer 220. Alternatively, the through-etchedwindow can be formed using a separate photolithography/etch step usingprocesses such as deep RIE.

Masking layer 220 is then removed, as shown in FIG. 17, where theexposed surface is then covered with a relatively thin oxide layer 224,as shown in FIG. 18, where a portion of this layer may be used todefined the evanescent coupling layer between a prism coupler andunderlying silicon waveguide. Layer 224 may be formed by using a thermaloxidation process. Other processes, such as atmospheric pressure CVD,low pressure CVD, plasma-enhanced CVD, evaporation or sputtering mayalso be used. Alternatively, other materials may be used to form thislayer, as discussed above, where other choices include silicon nitride,silicon oxynitride, silicon carbide. For processes such as thermaloxidation or low pressure CVD, the thickness and surface roughness oflayer 224 can be well controlled, an advantage for forming a preciselydefined evanescent coupling layer, as well as for bonding of a prismcoupler to an SOI wafer or die.

FIG. 19 illustrates the formation of an AR coating 226 on the patternedsurface of substrate 200 containing V-grooves 222. Exemplary materialsthat may be used for this coating include oxide-based and nitride-basedmaterials. The specifications for AR coating 226 will depend uponparameters such as the type of waveguide device used, the thickness ofwaveguide layer 32 (as well as the tolerances of these parameters), thediameter of the input optical beam and the wavelength range of interest.AR coating 226 may be formed prior to attaching the prism wafer to theSOI wafer, or subsequent to the attachment process. If AR coating 226 isformed prior to attachment, extra care must be taken to avoiddegradation of the evanescently coupled layer and/or attachment surfaceby stray depositions or scratches/particulates.

To physically attach the prism wafer (or die) to an SOI wafer includingan optical waveguide layer, processes similar to those described in theprior art may be used. That is, conventional attachment/bondingprocesses have been found sufficient to provide a permanent bond betweena prism coupler and SOI waveguide layer. For example, the coupler andSOI wafer may be pressed together and fused at a high temperature toform a permanent physical bond. Alternatively, a low temperature bondingprocess may be used. The use of a low temperature process permits theincorporation of device structures within the SOI wafer that cannotwithstand elevated temperatures. One exemplary process uses chemicallyactivated surfaces on both the prism and SOI wafer, allowing for lowtemperature bonding at a relatively moderate applied pressure. Thebonding chemistry must be compatible with the materials used in the SOIand prism wafers. Examples include, but are not limited to,compatibility with various integrated circuit metallizations systemsand/or AR coating materials. Regardless of the materials or processesused to attach the prism coupler to the SOI wafer, the actual attachmentmust be sufficient to withstand dicing/sawing operations used toseparate the various structures formed on a single wafer. Moreover, theattachment process should not cause any electrostatic damage to anyactive components on the SOI wafer. Another advantage of thesemiconductor-based bonding process is the alignment accuracy that canbe achieved between the two bonded substrates using integrated circuitmanufacturing infrastructures. For example, alignment accuracies betterthan ±1 μm can be routinely achieved using commercially available tools.It should be noted that both the prism coupler and the SOI wafer willhave alignment features in accordance with the intended application.

The coupling efficiency of the trapezoidal prism optical coupler of thepresent invention can be defined by determining the ratio of the powerin the output free-space beam relative to the power in the inputfree-space beam. Theoretically, for an embodiment with an evanescentcoupling layer of constant thickness, this ratio cannot exceed 80%,since the coupling efficiency is ultimately limited by the differentmode profiles exhibited by the input and output beams. Referring to FIG.20, the input beam is shown as having a Gaussian profile along thepropagation direction, while the beam from the output prism has anexponential profile along the propagation direction. While 80% couplingefficiency is too low to support some applications, it exceeds the valueassociated with other coupling techniques (for example, end-coupling orgrating coupling) and benefits from ease of manufacture. Thus, for thepurposes of discussion, it will be presumed that 80% coupling efficiencyis sufficient.

FIG. 21 contains a graph illustrating the coupling efficiency for thetrapezoidal coupling prism of FIG. 4, the efficiency illustrated as afunction of thickness of the evanescent coupling layer 38 for threedifferent thicknesses of waveguide layer 32. The magnitude of evanescentcoupling layer 38 is assessed for an application wavelength of 1550 nmand an input free space beam diameter of 63 μm. One exemplary thicknessof waveguide layer 32 is approximately 0.14 μm. The range of waveguidethickness shown in the graph of FIG. 21 (0.13–0.15 μm) represents theactual spread in layer thickness that is expected for state-of-the-artsilicon-on-insulator (SOI) processes. It is evident from the graph ofFIG. 21 that the evanescent coupling layer thickness must fall within±20 nm of the preferred value of 320 nm in order to avoid decreasing thecoupling efficiency by 10% (if the tolerance on the thickness of layer32, ±0.01 μm, is taken into account). This limitation corresponds toapproximately a ±6% tolerance in the evanescent coupling layerthickness, a value that is achievable by current state-of-the-artmanufacturing methods.

The efficiency limit can also be translated to a specific tolerance onthe flatness of the coupling layer, using the information contained inthe graph of FIG. 21. If the reduction in coupling efficiency by 10% isused as the criterion, then it is known that the gap should fall withinabout 300–340 nm for this example. For the configuration and preferredembodiment of FIG. 4, the largest dimension of the projection of theinput beam on the coupling surface is about 110 μm (for a free-spaceinput beam diameter of 63 μm). Thus, the maximum permitted wedge isabout 0.04 μm/100 μm=4×10⁻⁴ radians or 0.02°. It is to be noted that ifthe free-space beam diameter were closer to 180 μm, the permitted wedgewould be three times smaller, or 0.007°. Thus, the manufacturingtolerance on the permitted wedge of the evanescent coupling layer issignificantly eased by reducing the input beam diameter.

To improve the coupling efficiency of the prism coupling arrangement ofthe present invention, the mode profile of the output beam can be madeto more closely match that of the input beam. As the input beam from alaser source or input fiber will be Gaussian, the best way to improvethe coupling efficiency is to make the output profile be more similar tothat of a Gaussian beam. While the output beam in general will not betruly Gaussian, if the overlap integral of the new output beam with theinput Gaussian beam exceeds the overlap integral of the exponentialenvelope with the input Gaussian beam, the coupling efficiency can beimproved beyond 80%.

It is known from the prior art that one method of modifying the shape ofthe output beam is to grade the thickness of the evanescent couplinglayer along the propagation direction, as shown in the embodiment ofFIG. 9. That is, if the evanescent coupling layer retains an essentiallyconstant thickness, as shown in FIGS. 4–8, light will be coupled out ofwaveguide layer 32 and into prism 40 with the same coupling strength atall points, resulting in an output beam profile given by g(z)∝ exp(−αz).FIG. 20 contains graphs illustrating the thickness of the evanescentcoupling layer in the “input” and “output” areas above waveguide layer32, as well as graphs of the beam profile at the input and output. Asshown, the input beam is essentially Gaussian, and the output beamfollows the exponential relation as discussed above. The position of theinput comer 48 (see FIG. 4) is shown on the input beam graph and theposition of output comer 50 is shown on the output beam graph.

FIG. 22 illustrates the evanescent layer thickness profiles andinput/output beam shapes associated with using a tapered evanescentcoupling layer. If the output beam is to be more closely matched to theinput beam, the first light exiting from the prism surface should becoupled fairly weakly, so that most of the light remains in thewaveguide. To ensure that this occurs, the thickness of the evanescentcoupling layer should be well above the value required for optimumcoupling. After this point, the coupling strength seen by the lightneeds to increase so that the majority of light can be extracted to formthe peak of the “Gaussian”-like output beam. Thus, this portion of thebeam must sample the interface where the evanescent layer is close tothe optimal thickness. The majority of the energy is transferred fromwaveguide layer 32 and exits the system entirely through prism 40.Although the coupling strength continues to increase with the decreasingevanescent layer thickness, the amount of light exiting prism 40 startsto decrease as the light in the waveguide drops to lower and lowerlevels. In this manner, a more Gaussian-like profile for the output beamis achieved, as shown in FIG. 22. The optimum location of input andoutput beam coupling, with respect to evanescent layer thickness andbeam amplitude, is also shown in the graphs of FIG. 22.

While the invention concerns the design, geometry, construction, andattachment of a silicon-based prism die or wafer to an SOI device wafer,similar silicon-based prism dies or wafers can be attached in the samemanner to wafers that include waveguides formed from any material thathas a lower refractive index than silicon. Some exemplary waveguidematerials are indium phosphide, (n≈3.2 at 1550 nm) and lithium niobate(n≈2.1–2.2 at 1550 nm), although the choices are not limited to thesetwo materials. While such device wafers cannot be manufactured withstandard silicon processing techniques or features, applications forthese wafers could still benefit from the inventive silicon prismcoupler and attachment processes described here for optical input andoutput ports.

It is to be understood that the above-describes embodiments andprocesses of the present invention are exemplary only and should not beconsidered to define or limit the scope of the present invention asdefined by the following:

1. An optical coupling arrangement for providing a signal path into andout of a silicon optical waveguide formed in a surface layer of asilicon-on-insulator (SOI) wafer, the optical coupling arrangementcomprising a silicon-based prism coupler permanently attached to the SOIwafer in a manner such that a first, base surface of said prism coupleris disposed substantially parallel to, and mated with, an upperwaveguide surface of said SOI wafer, the prism coupler including acavity formed within the first, base surface wherein the refractiveindex of said silicon-based prism coupler at least equal to therefractive index of said silicon optical waveguide; and an evanescentcoupling region disposed between said silicon-based prism coupler andsaid silicon optical waveguide.
 2. An optical coupling arrangement asdefined in claim 1 wherein the thickness of the silicon opticalwaveguide is less than 1 μm.
 3. An optical coupling arrangement asdefined in claim 1 wherein the silicon optical waveguide is configuredto support propagation of a single mode optical signal.
 4. An opticalcoupling arrangement as defined in claim 1 wherein the silicon opticalwaveguide comprises a multi-layer structure of silicon-based layers,separated by relatively thin dielectric layers.
 5. An optical couplingarrangement as defined in claim 1 wherein a second, opposing surface ofthe silicon-based prism coupler is covered by an anti-reflective (AR)coating.
 6. An optical coupling arrangement as defined in claim 1wherein the silicon-based prism coupler and the silicon opticalwaveguide are formed to include dopants of a predetermined species andconcentration such that the refractive index of said silicon-based prismcoupler is equal to or slightly greater than the refractive index ofsaid silicon optical waveguide.
 7. An optical coupling arrangement asdefined in claim 1 wherein the evanescent coupling region comprises athin film layer of a material comprising a refractive index less thanthe refractive index of both the silicon-based prism coupler and thesilicon optical waveguide.
 8. An optical coupling arrangement as definedin claim 7 wherein the thin film layer evanescent coupling region isformed as a surface layer across the first, base surface of thesilicon-based prism coupler.
 9. An optical coupling arrangement asdefined in claim 7 wherein the thin film layer evanescent couplingregion is formed as a surface layer above the silicon optical waveguidelayer within the SOI wafer.
 10. An optical coupling arrangement asdefined in claim 7 wherein the thin film evanescent coupling regioncomprises a multi-layer structure.
 11. An optical coupling arrangementas defined in claim 10 wherein the multi-layer evanescent couplingregion comprises at least one layer formed across the first, basesurface of the silicon-based prism coupler and at least one layer formedas a surface layer of the SOI wafer.
 12. An optical coupling arrangementas defined in claim 7 wherein the evanescent coupling region comprises athin film of a material chosen from the group consisting of: silicondioxide, silicon nitride, silicon oxynitride and silicon carbide.
 13. Anoptical coupling arrangement as defined in claim 1 wherein theevanescent coupling region comprises a layer of relatively constantthickness.
 14. An optical coupling arrangement as defined in claim 1wherein the cavity is formed using an etching process to form abruptcorner edges along the region where the cavity meets the flat bottombase surface.
 15. An optical coupling arrangement as defined in claim 14wherein an RIE etching process is used to form the cavity withessentially vertical sidewalls.
 16. An optical coupling arrangement asdefined in claim 14 wherein an anisotropic wet chemical etching processis used to form the cavity with angled sidewalls.
 17. An opticalcoupling arrangement as defined in claim 1 wherein the evanescentcoupling region comprises a layer of tapered thickness so as to exhibita predetermined small thickness in regions where only a small portion ofincident light intensity is required to be transferred from thesilicon-based prism coupler to the silicon optical waveguide, saidthickness thereafter monotonically increasing as the fraction of lightintensity transferred from said silicon-based prism coupler to saidsilicon optical waveguide increases.
 18. An optical coupling arrangementas defined in claim 1 wherein the silicon-based prism coupler comprisesa single trapezoidal geometry, a first facet of said coupler defined asan input coupler and a second, opposing facet defined as an outputcoupler, wherein the trapezoidal flat bottom surface is defined as thefirst, base surface of said prism coupler, said flat bottom surfacedisposed substantially parallel to the associated SOI wafer andincluding a formed therein.
 19. An optical coupling arrangement asdefined in claim 18 wherein the surfaces of the at least one cavity arecoated with a material having a refractive index that is sufficientlylow so as to permit total internal reflection at the corner edges. 20.An optical coupling arrangement as defined in claim 1 wherein thesilicon-based prism coupler comprises a pair of trapezoidal prismssharing a base surface disposed substantially parallel to the SOI wafer,a first trapezoidal prism defined as an input prism and including aninput facet for use as an input coupler and a first cavity disposedalong the shared base surface thereof, and a second trapezoidal prismdefined as an output prism and including an output facet for use as anoutput coupler and a second cavity disposed along the shared basesurface thereof.
 21. An optical coupling arrangement as defined in claim1 wherein the silicon-based prism coupler is permanently attached to theSOI wafer by bonding the first, base surface of said silicon-based prismcoupler to an upper waveguide surface of said SOI wafer.
 22. An opticalcoupling arrangement as defined in claim 1 wherein the cavity within thebase surface of the silicon-based prism coupler is configured tofacilitate the attachment of said prism coupler to the surface of theSOI wafer.
 23. An optical coupling arrangement as defined in claim 22wherein the cavity is configured to provide a clearance above apatterned surface of the SOI wafer while also providing physical contactbetween the silicon-based prism coupler and the SOI wafer.